When aligning the wheels of a vehicle, it has been the common practice to mount one or more gauge heads to the wheels in order to measure certain angles such as camber, caster, toe and steering axis inclination. The gauge heads are pendulously supported by wheel clamps which are attached to the rims of the wheels so that the gauge head swings freely about the axis of rotation of the wheel. Any misalignment between the gauge axis and the wheel axis is known as runout and unless corrected for, results in inaccurate measurements and a subsequent misalignment of the wheels of the vehicle.
Pending application, Ser. No. 398,001, filed Aug. 24, 1989, now U.S. Pat. No. 4,953,296 and assigned to the assignee of the present application, discloses a method and apparatus for making a plurality of wheel alignment measurements using two or more angle sensors and for compensating such measurements for the runout between the angle sensors and the axis of rotation of the wheel on which the angle sensors are mounted. In the system described in that application, an encoder is mounted to the wheel for measuring and recording the angle of rotation of the wheel at a plurality of positions where runout measurements and subsequent wheel alignment parameter measurements are made by angle sensors.
U.S. Pat. Nos. 3,892,042, 4,138,825 and 4,192,074 also disclose methods and apparatus for providing runout compensation at the particular rotational position of the wheel at which the wheel alignment parameter measurements must be made. As disclosed in these patents, the technician must first rotate the wheel to a plurality of positions while the weight of the vehicle is lifted off the wheel and then make the actual wheel alignment parameter measurement at a particular position of rotation of the wheel while the weight of the vehicle is on the wheel. This requires the technician to use considerable care while making adjustments to the suspension system of the vehicle to avoid rotation of the wheel from the particular and only position where the runout can be corrected. The problem also occurs in relation to wheels on opposite sides of the vehicle which are connected by a differential, since rotation of the one wheel exerts a torque on the opposite wheel which will thus rotate from the preset position unless the technician takes the necessary action to prevent such unwanted rotation.